1. Field of the Invention
The present invention relates to an apparatus for forming a film, particularly to an apparatus for forming a film by the CVD method.
2. Description of the Related Art
There are various types of CVD apparatus for forming a film. With every CVD apparatus, the chemical reaction of a reaction gas is used for forming a film and, therefore, reaction products stick to the gas discharge surface of the gas distributor. These reaction products come off as particles and stick to the wafer, contaminating the wafer. To prevent the particles from sticking onto the wafer, it is necessary to regularly clean the gas discharge surface of the gas distributor of the CVD apparatus.
CVD apparatus of the plasma assisted type has the capability of reducing pressure in the chamber and of introducing an etching gas into the chamber at reduced pressure. Therefore, when reaction products stick to the gas discharge surface of such a gas distributor, it is possible to easily remove the reaction products from the gas discharge surface by introducing etching gas. This method is called in-situ cleaning and is used in practice.
An atmospheric pressure CVD apparatus using, for example, a reaction gas such as a SiH.sub.4 --O.sub.2 mixed gas or a TEOS-O.sub.3 mixed gas, however, because the apparatus does not normally have the capability of reducing the pressure in the chamber, when the reaction products stick to a gas discharge surface as powder particles, it is impossible to perform the in-situ cleaning as with the plasma assisted CVD apparatus. In such a case, the reaction products are physically removed by a cleaner or the like, or they are chemically removed by exposing the gas discharge surface to chemicals such as hydrofluoric acid (HF), after stopping the operation of the CVD apparatus and opening the chamber. Therefore, for the atmospheric pressure CVD apparatus, cleaning requires a lot of time and labor and this decreases the uptime/downtime ratio for the apparatus. Moreover, when the quantity of accumulated reaction products greatly increases, it becomes difficult to remove.